1. Field of the Invention
The present invention relates to a switching power supply and particularly relates to a switching power supply which reduces power consumption at no load and a light load and has an overload protecting function.
2. Description of Related Art
FIG. 10 is a circuit diagram showing an example of a conventional switching power supply. Each constituent block will be discussed below (e.g., Japanese Patent Laid-Open No. 5-30735).
In FIG. 10, a semiconductor device 130 for controlling a switching power supply is constituted of a switching element 101 and the control circuit thereof.
The semiconductor device 130 comprises, as external input terminals, five terminals of an input terminal (DRAIN) of a switching element 101, an auxiliary power supply voltage input terminal (VCC), an internal circuit power supply terminal (VDD), a feedback signal input terminal (FB), the output terminal of the switching element 101, and the GND terminal (GND) of the control circuit.
A regulator 102 for supplying an internal circuit power supply of the semiconductor device 130 comprises a switch 102A for supplying starting current to VCC and a switch 102B for supplying current from VCC to VDD.
A starting constant current source 103 for supplying starting circuit current supplies starting current to VCC via the switch 102A upon startup.
A start/stop circuit 107 for controlling start/stop of the semiconductor device 130 detects the voltage of VDD. When the voltage of VDD is equal to or lower than a certain voltage, the start/stop circuit 107 outputs a signal for stopping the switching operating of the switching element 101 to a NAND circuit 105.
A drain current detection circuit 106 detects an on voltage of the switching element 101, the on voltage being generated as a product of a current applied to the switching element 101 and an on resistance of the switching element 101, so that a current applied to the switching element 101 is detected. The drain current detection circuit 106 converts a detected current value of the switching element 101 into a voltage signal and outputs a voltage signal corresponding to the current value of the switching element 101 to a comparator 108.
In a feedback signal control circuit 111, a current signal inputted to the FB terminal is converted into a voltage signal and the signal is outputted to the comparator 108.
The comparator 108 outputs an “H” signal to an AND circuit 115 when the output signal from the feedback signal control circuit 111 and the output signal from the drain current detection circuit 106 are equal to each other.
An on blanking pulse generation circuit 114 outputs an “L” signal over a period of time since the “B” signal is inputted to the gate of the switching element 101, and outputs the “H” signal after the period of time. Since a current is detected by detecting the on voltage of the switching element 101, a current detection circuit detects a value larger than an actual current. This is because the on voltage does not sufficiently decrease despite that the actual current is low from when the switching element 101 is turned on to when the on voltage decreases. The on blanking pulse generation circuit 114 prevents such an erroneous detection.
The AND circuit 115 outputs a signal to the reset terminal of an RS flip-flop circuit 110 in response to the output signal of the comparator 108 and the input from the on blanking pulse generation circuit 114.
A clamping circuit 112 determines a maximum value of the output signal of the feedback signal control circuit 111. The maximum value determines the maximum value of current applied to the switching element 101 and provides overcurrent protection for the switching element 101.
An oscillator circuit 109 outputs a maximum duty cycle signal 109A for determining the maximum duty cycle of the switching element 101 and a clock signal 109B for determining an oscillation frequency of the switching element 101. The maximum duty cycle signal 109A is inputted to the NAND circuit 105 and the clock signal 109B is inputted to the set terminal of the RS flip-flop circuit 110.
The output signal of the start/stop circuit 107, the maximum duty cycle signal 109A, and the output signal of the RS flip-flop circuit 110 are inputted to the NAND circuit 105. The output signal of the NAND circuit 105 is inputted to a gate drive circuit 104 to control the switching operation of the switching element 101. Simultaneously the output signal of the gate drive circuit 104 is inputted to the on blanking pulse generation circuit to generate a blanking pulse signal.
A transformer 140 has a primary winding 140A, a secondary winding 140B, and a primary auxiliary winding 140C.
A rectifying/smoothing circuit constituted of a diode 131 and a capacitor 132 is connected to the primary auxiliary winding 140C and is used as an auxiliary power supply of the semiconductor device 130. The auxiliary power supply is inputted to VCC.
A capacitor 133 is inserted between the VDD terminal and GND to stabilize VDD.
A control signal transmission circuit 135 for transmitting a control signal from the secondary side to the primary side is constituted of a phototransistor 135A and a photodiode 135B. The collector of the phototransistor 135A is connected to FB and the emitter of the phototransistor 135A is connected to GND.
A rectifying/smoothing circuit constituted of a diode 150 and a capacitor 151 is connected to the secondary winding 140B. The photodiode 135B, a secondary-side control circuit 158, and a load 157 are connected to the rectifying/smoothing circuit.
The secondary-side control circuit 158 is constituted of a shunt regulator 152, resistors 154, 155, and 156, and a capacitor 153. Voltage divided by the detection resistors 154 and 155 of a secondary side output voltage VO is inputted to the reference of the shunt regulator 152, and a current applied to the photodiode 135B connected to the cathode of the shunt regulator 152 is controlled so as to have a constant secondary side output voltage VO.
Referring to FIGS. 10 and 11, the operations of the switching power supply configured thus will be described below. FIG. 11 is a time chart for explaining the operation waveforms of parts shown in FIG. 10.
In FIG. 10, a direct voltage VIN generated by, e.g., rectifying and smoothing a commercial AC power supply is inputted to the input terminal. VIN is applied to the DRAIN terminal of the semiconductor device 130 via the primary winding 140A of the transformer 140.
Then, starting current generated by a starting constant current source 103 is applied through the switch 102A in the regulator 102 to charge the capacitor 132 connected to VCC, so that the voltage of VCC increases. Since the switch 102B in the regulator 102 operates so that VDD has a constant voltage, the starting current partially charges the capacitor 133 connected to VDD via the switch 102B. Thus, the voltage of VDD also increases.
When VCC increases in voltage and reaches a starting voltage set in the start/stop circuit 107, the switching operation of the switching element 101 is started. When the switching operation is started, energy is supplied to the windings of the transformer 140 and thus current is applied to the secondary winding 140B and the primary auxiliary winding 140C.
Current applied to the secondary winding 140B is rectified and smoothed by the diode 150 and the capacitor 151 into direct current power and the power is supplied to the load 157.
Since the switching operation is repeated, the output voltage VO gradually increases. When the output voltage VO reaches a voltage set in output voltage detection resistors 154 and 155, a current applied to the photodiode 135B increases in response to a signal from the secondary-side control circuit 158.
Then, a current applied to the phototransistor 135A increases and a current applied from the FB terminal also increases.
When the current of the FB terminal (hereinafter referred to as IFB) increases, a voltage inputted to the comparator 108 (hereinafter referred to as VFBO) decreases and thus the drain current applied to the switching element 101 decreases. Such a negative feedback stabilizes the output voltage VO.
The current applied to the primary auxiliary winding 140C is rectified and smoothed by the diode 131 and the capacitor 132 and is used as the auxiliary power supply of the semiconductor device 130 and supplies current to the VCC terminal. Once VCC reaches the starting voltage, the switch 102A in the regulator 102 is turned-off. Thus, the current of the semiconductor device is supplied from the primary auxiliary winding 140C after startup. Since the primary auxiliary winding 140C has the same polarity as the secondary winding 140B, VCC is proportionate to the output voltage VO.
When an output current IO applied to the load 157 decreases after the output voltage VO is stabilized, IFB increases, VFBO decreases, and the drain current applied to the switching element 101 decreases. At this point, no matter how much the output current IO decreases, the drain current is not reduced to 0 and a small amount of drain current, which is determined by a blanking pulse outputted from the blanking pulse generation circuit, keeps flowing.
Further, when the output current IO applied to the load 157 increases, IFB decreases, VFBO increases, and the drain current applied to the switching element 101 increases according to the increase of IO. When VFBO increases and reaches a voltage set in the clamping circuit 112, overcurrent protection is performed and thus the drain current is clamped at a constant current ILIMIT.
However, in an overload state, even when the drain current of the switching element 101 is clamped, the output voltage VO decreases and the output current IO keeps increasing. Thus, an output current-voltage characteristic shown in FIG. 14 is obtained where the overload protection of the power supply does not sufficiently work.
FIG. 12 shows another conventional example where a power supply has an overload protecting function (e.g., Japanese Patent Application No. 2002-136674, which is unpublished and filed by the same applicant).
FIG. 12 is different from FIG. 10 in that an output current detection resistor 159, an overcurrent detection circuit 160, and an over current signal transmission circuit 136 are provided. In FIG. 12, when the output current IO reaches a certain value or higher, a current applied to a photodiode 136B increases, a current is applied from a power supply voltage VDD to GND via a phototransistor 136A, and the voltage of the VDD terminal decreases. Thus, a stop signal is outputted from a start/stop circuit 107, the switching operation of a switching element 101 is stopped, the overload protecting function of a power supply is performed, and an output current-voltage characteristic shown in FIG. 15 is obtained. The configuration of FIG. 12 cannot avoid an increase in the number of components.
As a conventional technique, the following method is available: a state of overload on the secondary side is detected by a voltage detection circuit composed of a shunt regulator and split resistors, an overload signal is transmitted by a photocoupler to the primary side, and the oscillation of an IC for switching control is stopped. For example, this configuration is disclosed in Japanese Patent Laid-Open No. 5-30735 in which current is applied from a photodiode connected to the terminal of an IC, so that the oscillation of the IC is stopped. However, this method requires another terminal only for stopping an overload.
Another conventional technique is a method disclosed in a patent document (Japanese Patent Laid-Open No. 2003-333843) in which a terminal other than an FB terminal for overload protection is provided so as to detect a reduced output voltage in an overload state, and a clamp voltage variable circuit reduces the oscillation frequency and the maximum current of a switching element, so that overcurrent protection is performed.
However, this method requires a terminal in addition to a feedback terminal, resulting in a complicated configuration.
In general, a switching power supply requires an overload protecting function. It is desired to minimize an output current in an overload state in order to prevent the parts of the switching power supply from liberating heat or being broken even when the overload state continues.
For this reason, the primary side generally has an overcurrent protecting function which prevents a current higher than a certain value from flowing to the switching element.
However, overcurrent protection only on the primary side cannot reduce an output current to a certain value or lower in the power supply.
Further, in order to solve this problem, another solution is necessary. For example, the output current and output voltage of the secondary side are detected and the switching operation of the primary side is stopped. Thus, the cost and the number of components are increased.
Moreover, the conventional structural examples cannot sufficiently reduce power consumption at no load and a light load.